As the scripture says, "Everyone who calls out to the Lord for help will be saved." But how can they call to him for help if they have not believed? And how can they believe if they have not heard the message? And how can they hear if the message is not proclaimed? And how can the mesage be proclaimed if the messengers are not sent out? - Romans 10:13-15

So, my questions are:

How does the volitional form work in this type of rhetorical question (without a か I might add)? Would the same meaning come across if these questions simply ended in 辞書形＋か (like どうして信じられるか)?

Can Verb-意志形 always be replaced by Verb-辞書形＋でしょう／だろう in this type of question? Because substituting that into these questions seems to make a little more sense to me. どうして信じられるでしょう → "How can they (possibly) believe?"

Of course, it does not make sense that you could substitute it when Verb-意志形 means "let's do X". (あとで外食しよう → "Let's go out to eat later", is clearly different than あとで外食するでしょう → "We'll probably go out to eat later")

I don't think you need to be defensive about posting Bible verses, anymore than you should be offended if someone asked questions about text from some other religious tradition. As long as it's a relevant question, there's no problem.
– AHelpsNov 3 '11 at 5:18

How does the volitional form work in this type of rhetorical question (without a か I might add)?

This kind of question often end in volitional form. （～しよう, ～だろう, etc.）

か is not absolutely needed if there is a question verb (どうして) in the sentence.

Would the same meaning come across if these questions simply ended in 辞書形＋か (like どうして信じられるか)?

～ようか sounds like you're thinking about this question, rather than asking someone else.

どうして信じられる(のだ/のか) can be a rhetorical question, but often requires a listener.

どうして信じられるか is used as a quoted question, どうして信じられるか、分からない

Can Verb-意志形 always be replaced by Verb-辞書形＋でしょう／だろう in this type of question? Because substituting that into these questions seems to make a little more sense to me. どうして信じられるでしょう → "How can they (possibly) believe?"

Yes, especially in modern Japanese.

Of course, it does not make sense that you could substitute it when Verb-意志形 means "let's do X". (あとで外食しよう → "Let's go out to eat later", is clearly different than あとで外食するでしょう → "We'll probably go out to eat later")

Hmm. I think the Japanese translation of Bible is a special kind of Japanese (翻訳調?). It's definitely not 文語調. But it is not the kind of 口語調 we usually speak.

I don't know if you can substitute it when it means "let's do", but I think it's sometimes substituted when it means “I will (agree to) do”.

FYI, There are multiple Japanese translations of the Bible just as there are multiple translations of other languages too. I usually use the [新共同訳]{しん・きょう・どう・やく} version, but there are also [口語訳]{こう・ご・やく} and [新改訳]{しん・かい・やく} versions that I'm aware of.
– istrasciApr 26 '14 at 20:23

@istrasci, oh, I searched であろう in the 新共同訳 version, and found a lot of unusual であろう. It might be a feature of translation. 創世記17章02節 わたしは、あなたとの間にわたしの契約を立て、あなたをますます増やすであろう / 出エジプト記04章23節 わたしの子を去らせてわたしに仕えさせよと命じたのに、お前はそれを断った。それゆえ、わたしはお前の子、お前の長子を殺すであろう
– Yang MuyeApr 26 '14 at 21:12